Off the top of my head there are a couple ways you could do it that don't require too much in the way of ne rules.
(1) You could just let them keep leveling as normal. The only odd thing you need to do is let the spell slot progressions keep rising because it'll be possible to get up to I think 65 prepared arcane levels or 72 spontaneous arcane or 30 divine levels from core rulebook PrCs alone if let them level up forever. Also you need to slap class levels on monsters because numbers are going to get huge. Wizards, Clerics, Druids, and Witches should probably get their first level 10 slots at level 20. You don't need spells for the higher level slots, just let them metamagic into them. Possibly change th Spell Perfection feat to allow metamagic up to 9 or your highest spell slot whichever is higher.
(2) You could overlap new class levels gestalt style. This keeps numbers under control and while the CR system will warp a bit it won't completely break. As a first approximation I'd treat the party as 5% larger for every gestalted level. To take a complicated example suppose you have a fighter 1/wizard 9/eldritch knight 10 in that order with the trait for +2 caster levels to a maximum of your total level. At level 21 you choose something to overlap with fighter 1. Suppose you choose wizard. Your total wizard casting is now level 19. You get your 2 free spells up to the level you can currently cast. Your BAB doesn't go up because you already had a +1 BAB at level 1. Your hit points don't change because 1d6 cannot be better than the maximized 1d10 you already have at this level. You get your level 10 wizard feat and can qualify for it with your 20 caster levels 10 wizard levels, and everything else you have at level 20. You get the +1 will save from line 10 on the wizard table. Your caster level does not go up to 21 because you still only have 20 levels, the first one is just gestalted now. Then you take fighter levels over the wizard levels. At each of these where wizard gave no BAB your BAB now goes up. You lose a 1d6 hit die and gain a 1d10. You get your fighter class features and bonus feats. You take wizard 12 over eldritch knight 1 because that one doesn't give casting progression, getting you up to 20 levels of wizard spellcasting and you get your level 12 school stuff. Over EK2 you could take a wizard level, but you already have a level of wizard casting at that level so you'd only get the 2 free spells and any level 13 wizard abilities, which appears to amount to +1 int and natural armor for your familiar and the ability to scry on it. Let's say you really want this and take that level. Then you're pretty much done gishing so let's take some rogue for the sneak attack, and because every class involved so far has had the same number of skill points. Rogue 1 is going to add +2 to your reflex save, +6 skill points (because EK already had 2), and sneak attack.
3) You could just take new levels but cap BAB and caster level at 20, saving throws at 12, and such. The end results are similar to gestalting, but the gains are faster early and will run into a wall so you may want to treat the party as 10% larger per level past 20 for CR and cap at something like 30 total levels.
